do you have something like craig list in London? On the local news they have been reporting that there have been a lot of reports of people finding their stuff being sold on that way and contacting the police who then do a sting op. Or alternately they (the victim)pretend to be a buyer so they can check it out before calling the police.

Maybe you need to get security cameras installed if they are brazen enough to go into your garden. Only one step more and they would be in your house.